Efficient conversion of propane in a microchannel reactor at ambient conditions

The oxidative dehydrogenation of propane, primarily sourced from shale gas, holds promise in meeting the surging global demand for propylene. However, this process necessitates high operating temperatures, which amplifies safety concerns in its application due to the use of mixed propane and oxygen. Moreover, these elevated temperatures may heighten the risk of overoxidation, leading to carbon dioxide formation. Here we introduce a microchannel reaction system designed for the oxidative dehydrogenation of propane within an aqueous environment, enabling highly selective and active propylene production at room temperature and ambient pressure with mitigated safety risks. A propylene selectivity of over 92% and production rate of 19.57 mmol mCu−2 h−1 are simultaneously achieved. This exceptional performance stems from the in situ creation of a highly active, oxygen-containing Cu catalytic surface for propane activation, and the enhanced propane transfer via an enlarged gas-liquid interfacial area and a reduced diffusion path by establishing a gas-liquid Taylor flow using a custom-made T-junction microdevice. This microchannel reaction system offers an appealing approach to accelerate gas-liquid-solid reactions limited by the solubility of gaseous reactant.

publicafion.1.The authors have claimed that the dissolufion of Cu may be responsible for the C-H acfivafion.What is the relafionship between propane conversion vs. Cu dissolufion rate? 2. It should be clarified that whether Cu2O or soluble Cu2+ is the working species?This issue is very important regarding to the explanafion of reacfion mechanism.Because in the theorefical calculafion, the authors employed Cu2O as the model surface, which could be totally wrong.
3. The authors have provided the stability of propane conversion within a 12 h operafion in the Cu microtube reactor at room temperature.It seems that the propane conversion kept unchanged.My quesfion is what is the stability of such Cu microtube.According to the dissolufion rate, the loss of Cu could be as high as 25.6 gCu/(m2*h) under O2 parfial pressure of 0.2 atm.Therefore, the stability of reactor under long-term operafion remains uncertain.
Reviewer #1 (Remarks to the Author): In this manuscript, the authors demonstrated a microchannel reactor for improving propane partial oxidation to propene under room temperature.The reported propene production rate is comparable to those achieved at elevated temperatures/pressure.Additionally, the authors provided mechanistic insights based on the reaction order analysis and DFT calculations.Beyond enhancing propane conversion, this innovative reactor design has implications for improving mass transport in other gas-liquid reaction systems.While this manuscript is well written and can be recommended for publication, the authors are advised to address a few minor concerns: We thank Reviewer 1 for the positive appraisal.
1.The authors should confirm that the flow pattern remains consistent at the microtube reactor's outlet, especially considering the substantial consumption of reactants like O2, which might alter the flow pattern.
Response: The flow patterns at the reactor's outlet maintained the characteristics of Taylor flow, despite the consumption of O2 during the reaction (Figure R1).It is noted that the consumption of O2 slightly reduces the length of the gas bubbles.Action: We added the following sentences in the 1 st paragraph on page 7: "We note that the flow patterns at the reactor's outlet maintained the characteristics of Taylor flow, with slightly reduced length of gas bubbles due to the consumption of O2 during the reaction (Supplementary Figure 7)." We added the following figure to the Supplementary Information as Supplementary Figure 7: Supplementary Figure 7.Comparison of the high-speed camera photographs of the Taylor flow at inlet and outlet with various liquid flow rates (L).The gas flow rate was fixed at 2 mL min -1 .
2. A justification for the selection of the Taylor flow over other flow patterns, such as bubbly flow, is needed.
Response: The reason for selecting the Taylor flow over other flow patterns is primarily due to the two simultaneously achieved advantages: significantly enhanced the gas-liquid interfacial area and effectively reduced the liquid film thickness, thereby facilitating the molecular diffusion of gaseous reactants.These factors are crucial for improving the mass transport of the gaseous reactants in propane activation.In addition, Taylor flow is characterized for its uniformity, making it suitable for conducting mass transport characteristic analyses.In contrast, other flow patterns such as bubbly flow and annular flow as shown in Figure R2, have limitations: the bubbly flow, while increasing the interfacial area, does not reduce the diffusion path, and the annular flow, with an extremely low liquid to gas ratio, does not provide a sufficient liquid supply necessary for propane activation.Action: We added the following sentences in the 2 nd paragraph on page 3: "In addition, Taylor flow is characterized for its uniformity, making it suitable for conducting mass transport characteristic analyses.Other flow patterns such as bubbly flow and annular flow (Supplementary Figure 3), have limitations in propane activation: the bubbly flow, while increasing the interfacial area, does not reduce the diffusion path, and the annular flow, with an extremely low liquid to gas ratio, does not provide a sufficient liquid supply necessary for propane activation." We added the following figure to the Supplementary Information as Supplementary Figure 3:   Action: We added the following sentences in the 2 nd paragraph on page 3: "The morphology and chemical composition of Cu microtube were characterized using scan electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) (Supplementary Figure 1)" We added the following figure to the Supplementary Information as Supplementary Figure 1:

Figure R1 .
Figure R1.Comparison of the high-speed camera photographs of the Taylor flow at inlet and outlet with various liquid flow rates (L).The gas flow rate was fixed at 2 mL min -1 .

Figure R2 .
Figure R2.Comparison of flow pattern of (a) Taylor flow, (b) bubbly flow and (c) annular flow.

Supplementary Figure 3 .
Comparison of flow pattern of (a) Taylor flow, (b) bubbly flow and (c) annular flow.3. Including an illustrative scheme or photograph of the T-junction would enhance clarity about how the Taylor flow originates.Additionally, a comprehensive schematic of the entire setup would be beneficial for readers.Response: Thanks for the comment.A scheme of the entire setup as well as the T-junction is provided in FigureR3.

Figure R3 .
Figure R3.Scheme of the experimental setup for propane activation in Cu microchannel reactor.

Figure R4 .
Figure R4.Photo and SEM image of Cu microtube.(a) A photo of cross section, (b-d) SEM image (b) with the elemental distribution by EDX for Cu (red) (c) and O (cyan) (d).